In a public venue such as a large auditorium, excessive reverberation negatively impacts the sound quality of audio material being delivered to an audience. Reverberation is the persistence of sound in a particular space after the original sound dissipates. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air. If the ratio of source audio to reverberation is low enough, the source material is masked by the presence of the reflected/reverberant audio. This leads to degradation in the quality of the presented source material, leading to difficulty in understanding speech. If the reverberation time is too long, the presented material sounds ‘muddy’. These problems are exacerbated in larger auditoriums, since the larger the auditorium, the longer the delay and the greater the reverberation time.
Ideally, an auditorium should be designed for good acoustics. The shape of the auditorium is a major factor in the ratio of direct to reflected sound arriving at the audience. Sound absorbing material on the walls and ceilings can reduce the amount of sound reflected around an auditorium. Diffuse reflectors placed around the ceilings and walls help prevent the reflections from being too coherent, so that an ‘echo’ is not perceived.
However, relatively few public venues are actually designed for ‘good’ acoustics. Music concerts and other public events are usually held in the largest venue possible such as sports arenas because the performer typically wishes to sell as many tickets, and fill as many seats, as possible. However, in these large venues there is typically a severe degradation of the perceived audio due to the delayed reflections and reverberations of the source audio from the walls and ceilings.
One of the simplest techniques of reducing the effects of auditorium reverberation is to elevate the loudspeakers and point the sound in the direction of the audience, and away from the ceiling of the auditorium. This reduces the amount of sound pointed directly at a reflective ceiling and back wall, reducing reflection and hence, reverberation, from these surfaces.
However, this is not always possible, especially in the case of a large venue. Further, if the sound source is perceptively disjointed with the source (singer, speaker, etc . . . ), this can have a negative impact on the audience.
Advancements in electronics have lead to the development of phased array loudspeakers, wherein the timing of sounds arriving at the various loudspeakers in the array allow the source audio to be ‘steered’ towards the audience.
Another approach is to place loudspeakers throughout the auditorium, pointed at the audience. The human acoustic perception of the source of sound is maintained by delaying the sound at remote loudspeakers so that it arrives at the audience within milliseconds after the sound from the stage.
However, these solutions are not always practical, given the shape and size of a particular venue.
Another approach is based on active sound cancellation, in which a particular sound or noise is monitored and a phase-shifted sound is generated to cancel or reduce the unwanted sound or noise.
For example, U.S. Pat. No. 5,363,451 describes a system and method for cancelling noise in an open space from a particular noise source such as a ‘machine’. The system detects noise at the noise source and cancels it at another point.
U.S. Pat. No. 5,699,437 describes a system that employs arrays of microphones to pick up ‘noise’ emanating from some point inside an auditorium and plays the ‘opposite sound’ or ‘cancelling noise’ from loudspeakers situated around the auditorium such that the noise is cancelled at some optimal location in the auditorium (the audience). Note that this solution can improve the sound at a ‘sweet spot’ in the auditorium, but it can also result in detrimental performance in another area of the same auditorium. Further, this system needs tuning by an expert on installation.
U.S. Pat. No. 6,795,557 describes a method for reducing the level of acoustical reflections in a room by attempting to cancel reflected signals as they go into the intended ‘quiet area’. However, at this point it becomes difficult to distinguish between reverberations and source audio, particularly where the room or venue has multiple reflective surfaces where the reverberations are not coherent.